Each of Japanese Patent No. 3856215 and Japanese Patent Application Laid-open No. 2008-299573 discloses, for example, a configuration to make it possible to increase a control gain of a speed control loop in a motor control device by compensating a phase delay of a vibrational component in the speed control loop using a phase compensating speed observer. The phase compensating speed observer is provided, on the front stage side, with an equivalent rigid body model that assumes a controlled object as a rigid body, and, on the rear stage side, with a delay element model simulating a delay element that exists in a controller at a stage later than a PI controller or that exists in the controlled object. The equivalent rigid body model and the delay element model are connected in series to each other. The phase compensating speed observer supplies an output of the PI controller to the equivalent rigid body model, and also feeds back a signal obtained by subtracting an output of the delay element from a speed of the controlled object to an integral element through multiplication by a stabilizing gain. Then, the phase compensating speed observer outputs, as a speed feedback signal of the speed control loop, an input entered into the delay element model.
The phase compensating speed observer disclosed in Japanese Patent No. 3856215 mentioned above is provided with a delay element model including only one integral element (1/s), that is, a first-order delay element model whose order is one. The phase compensating speed observer disclosed in Japanese Patent Application Laid-open No. 2008-299573 mentioned above is provided with a fourth-order delay element model in which the order of the integral element (1/s) is estimated to be high by exactly simulating the delay element existing in the controlled object.
If the order of the integral element in the delay element model is too low as disclosed in Japanese Patent No. 3856215 mentioned above, a problem occurs that the phase of a fed-back speed leads too much ahead of that of a speed reference. If, in contrast, the order of the integral element in the delay element model is too high as disclosed in Japanese Patent Application Laid-open No. 2008-299573, when the observer is actually implemented as software, problems occur in that the observer spends redundant processing time and the control gain can be set only within a narrow range.